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A synchronization point at which threads can pair and swap elements within pairs. Each thread presents some object on entry to theexchange
method, matches with a partner thread, and receives its partner‘s object on return. An Exchanger may be viewed as a bidirectional form of aSynchronousQueue. Exchangers may be useful in applications
such as genetic algorithms and pipeline designs.
JDK1.5中有一个Exchanger类，可以用来完成线程间的数据交换。
类java.util.concurrent.Exchanger提供了一个同步点，在这个同步点，线程可以组对，并且彼此交换数据。一个Exchanger可以看作是SynchronousQueue的双向版本（SynchronousQueue本身数据是单向的）,每个线程通过exchange()方法的入口提供数据给他的伙伴线程，并接收他的伙伴线程提供的数据，并返回。当两个线程通过Exchanger交换了对象，这个交换对于两个线程来说都是安全的。 Sample Usage: Here are the highlights of a class that uses anExchanger to swap buffers between threads so that the thread filling
the buffer gets a freshly emptied one when it needs it, handing off the filled one to the thread emptying the buffer.
class FillAndEmpty {
Exchanger exchanger = new Exchanger();
DataBuffer initialEmptyBuffer = ... a made-up type
DataBuffer initialFullBuffer = ...
class FillingLoop implements Runnable {
public void run() {
DataBuffer currentBuffer = initialEmptyBuffer;
try {
while (currentBuffer != null) {
addToBuffer(currentBuffer);
if (currentBuffer.isFull())
currentBuffer = exchanger.exchange(currentBuffer);
}
} catch (InterruptedException ex) { ... handle ... }
}
}
class EmptyingLoop implements Runnable {
public void run() {
DataBuffer currentBuffer = initialFullBuffer;
try {
while (currentBuffer != null) {
takeFromBuffer(currentBuffer);
if (currentBuffer.isEmpty())
currentBuffer = exchanger.exchange(currentBuffer);
}
} catch (InterruptedException ex) { ... handle ...}
}
}
void start() {
new Thread(new FillingLoop()).start();
new Thread(new EmptyingLoop()).start();
}
}
}
算法介绍：
/*
* Algorithm Description:
*
* The basic idea is to maintain a "slot", which is a reference to
* a Node containing both an Item to offer and a "hole" waiting to
* get filled in. If an incoming "occupying" thread sees that the
* slot is null, it CAS'es (compareAndSets) a Node there and waits
* for another to invoke exchange. That second "fulfilling" thread
* sees that the slot is non-null, and so CASes it back to null,
* also exchanging items by CASing the hole, plus waking up the
* occupying thread if it is blocked. In each case CAS'es may
* fail because a slot at first appears non-null but is null upon
* CAS, or vice-versa. So threads may need to retry these
* actions.
*
* This simple approach works great when there are only a few
* threads using an Exchanger, but performance rapidly
* deteriorates due to CAS contention on the single slot when
* there are lots of threads using an exchanger. So instead we use
* an "arena"; basically a kind of hash table with a dynamically
* varying number of slots, any one of which can be used by
* threads performing an exchange. Incoming threads pick slots
* based on a hash of their Thread ids. If an incoming thread
* fails to CAS in its chosen slot, it picks an alternative slot
* instead. And similarly from there. If a thread successfully
* CASes into a slot but no other thread arrives, it tries
* another, heading toward the zero slot, which always exists even
* if the table shrinks. The particular mechanics controlling this
* are as follows:
*
* Waiting: Slot zero is special in that it is the only slot that
* exists when there is no contention. A thread occupying slot
* zero will block if no thread fulfills it after a short spin.
* In other cases, occupying threads eventually give up and try
* another slot. Waiting threads spin for a while (a period that
* should be a little less than a typical context-switch time)
* before either blocking (if slot zero) or giving up (if other
* slots) and restarting. There is no reason for threads to block
* unless there are unlikely to be any other threads present.
* Occupants are mainly avoiding memory contention so sit there
* quietly polling for a shorter period than it would take to
* block and then unblock them. Non-slot-zero waits that elapse
* because of lack of other threads waste around one extra
* context-switch time per try, which is still on average much
* faster than alternative approaches.
*
* Sizing: Usually, using only a few slots suffices to reduce
* contention. Especially with small numbers of threads, using
* too many slots can lead to just as poor performance as using
* too few of them, and there's not much room for error. The
* variable "max" maintains the number of slots actually in
* use. It is increased when a thread sees too many CAS
* failures. (This is analogous to resizing a regular hash table
* based on a target load factor, except here, growth steps are
* just one-by-one rather than proportional.) Growth requires
* contention failures in each of three tried slots. Requiring
* multiple failures for expansion copes with the fact that some
* failed CASes are not due to contention but instead to simple
* races between two threads or thread pre-emptions occurring
* between reading and CASing. Also, very transient peak
* contention can be much higher than the average sustainable
* levels. An attempt to decrease the max limit is usually made
* when a non-slot-zero wait elapses without being fulfilled.
* Threads experiencing elapsed waits move closer to zero, so
* eventually find existing (or future) threads even if the table
* has been shrunk due to inactivity. The chosen mechanics and
* thresholds for growing and shrinking are intrinsically
* entangled with indexing and hashing inside the exchange code,
* and can't be nicely abstracted out.
*
* Hashing: Each thread picks its initial slot to use in accord
* with a simple hashcode. The sequence is the same on each
* encounter by any given thread, but effectively random across
* threads. Using arenas encounters the classic cost vs quality
* tradeoffs of all hash tables. Here, we use a one-step FNV-1a
* hash code based on the current thread's Thread.getId(), along
* with a cheap approximation to a mod operation to select an
* index. The downside of optimizing index selection in this way
* is that the code is hardwired to use a maximum table size of
* 32. But this value more than suffices for known platforms and
* applications.
*
* Probing: On sensed contention of a selected slot, we probe
* sequentially through the table, analogously to linear probing
* after collision in a hash table. (We move circularly, in
* reverse order, to mesh best with table growth and shrinkage
* rules.) Except that to minimize the effects of false-alarms
* and cache thrashing, we try the first selected slot twice
* before moving.
*
* Padding: Even with contention management, slots are heavily
* contended, so use cache-padding to avoid poor memory
* performance. Because of this, slots are lazily constructed
* only when used, to avoid wasting this space unnecessarily.
* While isolation of locations is not much of an issue at first
* in an application, as time goes on and garbage-collectors
* perform compaction, slots are very likely to be moved adjacent
* to each other, which can cause much thrashing of cache lines on
* MPs unless padding is employed.
*
* This is an improvement of the algorithm described in the paper
* "A Scalable Elimination-based Exchange Channel" by William
* Scherer, Doug Lea, and Michael Scott in Proceedings of SCOOL05
* workshop. Available at: http://hdl.handle.net/1802/2104
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原文地址：http://blog.csdn.net/aigoogle/article/details/38494363
